The present invention generally relates to amplifier systems, and more particularly to a split-band amplifier system which takes advantage of the inherent redundancy of such systems to provide an amplifier system having increased reliability without the need for additional amplifiers.
Split-band amplification is a technique wherein a plurality of amplifiers are connected in parallel with each amplifier being assigned a limited portion of a predetermined pass band for amplification. In other words the pass band of the amplifier system is divided into a plurality of contiguous split-bands by filters, and the outputs of the filters are connected to respective ones of the amplifiers for amplification of signals within the individual split-bands. The outputs of the amplifiers are then combined to provide a single, amplified output covering the entire pass band. There are many applications of split-band amplification ranging from audio to microwave frequency bands. In general, split-band amplification may be used whenever the frequency band of the signals to be amplified is relatively wide. Obviously, this technique would be used if the frequency band exceeds the bandwidth of the individual amplifiers that would be used in the system, but the technique is often used even if the frequency band is well within the bandwidth of the individual amplifiers in order to avoid intermodulation products. Such intermodulation products can occur when signals of substantially different frequencies are amplified in the same amplifier and represent a degradation of the original signal.
One application of split-band amplification is in the field of CATV systems. As the art is being advanced to include a greater number of channels, a very serious problem is encountered which causes a rather substantial cost increase in the implementation of such systems. Cable television is a system of signal distribution which to this point in time was capable of transporting a plurality of television signals for a reasonable distance while maintaining intermodulation distortion characteristics invisible to the most scrutinous viewer. Over the years, a larger number of channels was being demanded and amplifier linearity was sufficiently improved so that overall, these distortion products remained essentially below the visibility threshold with no significant change to the usable cascade length. The latest move to 52 channels makes linearity demands on the amplifying system which at this point are not available. As a result, the length of a single cascade is substantially reduced which forces the inclusion of more signal origination points than are economically viable. Through the utilization of split-band amplifiers the channel loading or discrete carriers, as the case may be, are shared thereby providing a means to again make cascades of such wideband circuits feasible while maintaining high quality signals. Split-band amplifiers will, of course, provide the desired high quality signal, but amplifiers in a CATV system are distributed along the cable and, therefore, not immediately accessible in the event of an amplifier failure. Typically, reliability is provided by building redundancy into the system. In other words, in the case of the failure of an amplifier, a redundant amplifier is available to be automatically switched into the circuit in place of the failed amplifier. The provision of redundant amplifiers greatly increases the reliability of the system; however, this reliability is at the expense of greatly increased cost and complexity, and in the case of split-band amplification systems, the expense is often prohibitive. As a result, there is often the necessity of making a choice between high quality amplification and reliability.
Another application of split-band amplification is in the field of satellite communication systems. Because of the very high costs of the satellite transponders, it is mandatory that both high quality amplification and good reliability be built into these devices. However, there are other factors which must also be considered, chief among which are space and weight.
It is therefore an object of this invention to provide a split-band amplifier system which takes advantage of the inherent redundancy of such systems to provide increased reliability without the need for additional amplifiers.
It is a more specific object of the invention to provide a highly reliable amplifier system for use in a CATV system, which amplifier system is capable of high quality, wideband amplification permitting cascading of such amplifier systems over considerable distances.